Field of the Invention
The invention relates to a honeycomb body, through which a fluid can flow along an axis and through a multiplicity of channels in the honeycomb body.
Such honeycomb bodies find multiple uses as carrier bodies for catalysts that are intended for the catalytic conversion of reactable components of fluids. One significant field of application for honeycomb bodies with catalysts is catalytic scrubbing of exhaust gases from internal combustion engines, particularly engines in motor vehicles. To that end, catalytically coated honeycomb bodies are installed in the exhaust systems of the engines, and the exhaust gases produced during engine operation flow through them.
The honeycomb bodies are produced from ceramic compositions or from sheet-metal layers. In order to form a metal honeycomb body, sheet-metal layers, at least some of which are corrugated, folded or otherwise structured, are layered, stacked, spirally wounded or intertwined in some other way. Options therefor are described in European Pat. No. 0 223 058 B1, corresponding to U.S. Pat. No. 4,824,011; European Patent No. 0 245 736 B1; European Patent No. 0 245 737B1, corresponding to U.S. Pat. Nos. 4,832,998 and 4,923,109; and European Patent No. 0 245 738 B1, corresponding to U.S. Pat. Nos. 4,803,189 and 4,946,822; U.S. Pat. Nos. 4,753,981 and 4,822,766; Published International Applications WO 89/10470 A1, WO 89/10471 A1, WO 90/03220 A1, and WO 90/12951 A1; and German Published, Non-Prosecuted Application DE 39 03 879 A1, as well as German Petty Patent No. 89 08 738 U1.
Catalysts for the conversion of reactable components of a fluid flowing around them typically do not develop their catalytic effect until above certain limit temperatures which are specific to the particular catalyst and to the reaction to be catalyzed, that is so-called "light-off temperature". In the case of catalysts intended for converting pollutants in the exhaust gases of typical internal combustion engines, the light-off temperatures are generally on the order of several hundred degrees Celsius. Therefore, such a catalyst must be heated if it is to become active. In a motor vehicle exhaust system, that is done as a rule by the gas flowing through the honeycomb body coated with the catalyst, but the catalytic effect ensues only with a certain delay after the engine is put into operation. Electric preheating of the catalyst or of the honeycomb body carrying it is already known in order to eliminate or reduce that delay. Published International Applications WO 89/10470 A1, WO 89/10471 A1, WO 90/03220 A1, and WO 90/12951 A1; and German Published, Non-Prosecuted Application DE 39 03 879 A1, as well as German Petty Patent No. 89 08 738 U1, provide teaching on that point. The layers forming the metal honeycomb body are electrically interconnected in such a way that a path is formed to guide an electric current through the honeycomb body. Current supply leads are also mounted on the honeycomb body, to which an electric current source, for instance a motor vehicle battery, should be connected through suitable switchgear and lines. A honeycomb body of a typical size and structure for use in the exhaust system of a motor vehicle requires a heating capacity of from several hundred watts to more than 4 kW for heating in a sufficiently short time. Therefore, an onboard electrical network with a voltage of 12 V, as is usual in passenger cars, would have to furnish currents of up to 400 A to heat the honeycomb body. The electrical resistance of the honeycomb body must correspond to the electrical voltage available and to the heating capacity to be brought to bear. However, metal honeycomb bodies of the typical type have electrical resistances of only a few thousandths of an ohm at most. Such a honeycomb body would draw currents of up to 1000 A from an electrical voltage source with a voltage of approximately 12 V, meaning that a typical motor vehicle battery would be strained to a virtually unsupportable extent. From the prior art documents cited above, provisions are already known for increasing the electrical resistance of a honeycomb body for given geometrical dimensions. For instance, electrically heatable honeycomb bodies may be split by means of gaps and/or electrically insulating partitions between the layers in such a way that a current path having an electrical resistance of a suitable level is produced and extended through the honeycomb body. The suggestion has also already been made of using not a single honeycomb body with a catalyst coating but rather two such honeycomb bodies. An electrically heatable smaller honeycomb body precedes a larger honeycomb body that is not intended to be heated directly and both honeycomb bodies have substantially the same diameter, but the smaller honeycomb body is substantially shorter than the larger honeycomb body. Due to the dimensioning, a significantly higher electrical resistance can already be achieved in the smaller honeycomb body as compared with the larger honeycomb body. In particular, the smaller honeycomb body can be constructed in such a way that with limited strain on a voltage source, it can be brought rapidly enough to a temperature above the light-off temperature if the catalyst located on the honeycomb body. The typically exothermic catalytic reaction that ensues upon the exhaust gas flowing through the smaller honeycomb body causing heating of the exhaust gas and thus reinforces the heating of the larger honeycomb body, which in the final analysis takes on the main burden of the catalytic converter. In German Petty Patent No. 89 08 738 U1, metal plates which are intended for producing an electrically heated honeycomb body may be provided with holes, in order to increase their specific resistance and to obtain a honeycomb body with a relatively high electrical resistance. Published International Application WO 90/12951 A1 shows options for mechanically loadable insulations in honeycomb bodies.
The configuration described above having a smaller honeycomb body and a larger honeycomb body, with the smaller honeycomb body disposed upstream of the larger honeycomb body in the exhaust system, is also advantageous even if separate heating is not provided for. The smaller honeycomb body intrinsically has a lower thermal capacity than the larger honeycomb body and therefore warms up relatively quickly if a hot fluid, such as exhaust gas, flows through it. Therefore, in the smaller honeycomb body, a catalyzed reaction also comes rapidly into play and with the heat released reinforces the heating of the larger honeycomb body.
Both considerations regarding electrically heating the smaller honeycomb body and considerations regarding how to dimension its thermal capacity lead to keeping the mass of the smaller honeycomb body small, but naturally with the secondary condition that the smaller honeycomb body, after the activation of the catalyst located on it, furnishes adequate heating of the fluid flowing through it and reinforces the heating of the larger honeycomb body. Even in view of that secondary condition, those considerations may possibly contradict the requirements for mechanical stability of the smaller honeycomb body: particularly in a motor vehicle exhaust system, severe pulsation occurs, and the honeycomb bodies are exposed to considerable mechanical strains, which is why stringent requirements must be made of them in terms of strength.